Grasses, the plant family Gramineae, first showed up in the Early Cenozoic, specifically i

Grasses, the plant family _Gramineae_, first showed up in the
Early Cenozoic, specifically in the Paleogene (sometime in the
Early Paleocene). They did not reach their full ecological
potential until the Late Oligocene and Miocene, however. Early
grasses were apparently confined to wooded or swampy areas.
Like modern sedges that form the marshlands along coastal
areas, the mode of growth of early grasses did not allow their
leaves to grow continuously and thus to recover from heavy
grazing by animals of the type that inhabit open country in
large numbers. It was only by virtue of an adaptive
breakthrough - the origination of continuous growth - that
grasses were ultimately able to invade open country with great
success. Once they were able to survive the effects of heavy
grazing by animals, grasses spread quickly over vast expanses
of the Earth to form grasslands.
Since you asked (remember, you DID ask...), there is another
interesting evolutionary point to consider in the development of
grasses: with the enormous numbers of individual grass plants
populating marshes and grasslands, effective reproduction would
be damned high impossible if these plants required pollination
by insects. Thus, it comes as no surprize that grasses are
wind-pollinated (much to the chagrin of allergy sufferers)
plants. But, it should be noted that grasses can carpet large
areas by means of budding (through tillers and rhizomes), a
non-sexual reproductive strategy in which individuals send out
subterrestrial runners that sprout new plants, thus rapidly and
densely colonizing and stabilizing the soil and driving to
distraction homeowners who must deal with crabgrass...